1. Field of the Invention
The present invention relates to a control device which controls a controlled variable of a multiphase electric rotating machine by controlling a power inverting circuit having a plurality of switching elements such that each switching element electrically connects or disconnects one of a plurality of voltage applying portions, applying different voltages, to or from one of terminals of the machine.
2. Description of Related Art
A control device for a three-phase motor has been used. For example, this device performs a pulse width modulation (PWM) control, based on triangular wave comparison, for the motor to control three phase currents, actually flowing through respective phase windings of the motor, to an instructed value under the feed-back control. In this PWM control, an instructed value of phase voltages to be applied from an inverter to the respective phase windings of the motor is calculated, and switching elements of the inverter are operated based on the difference between the level of a carrier signal shaped in a triangular wave and the instructed value.
However, when this PWM control is performed in an excessive modulation region in which the instructed voltage value is higher than an input voltage of the inverter, levels of higher harmonic waves contained in the output voltage of the inverter increase. In this case, the higher harmonic waves undesirably influence the response characteristics of the current flowing through the phase windings of the three-phase motor. This problem is arisen because it is assumed in the design of a current control system that the output voltage of the inverter can be always set at the instructed value.
To avoid this problem, Published Japanese Patent First Publication No. 2008-228419 proposes a model prediction control performed in a control device. In this control, a plurality of operating states of an inverter applying a voltage to a three-phase motor are preset, a current expected to flow through the motor is predicted from the output voltage of the inverter, set in one operating state, for each of the operating states, and the inverter is actually set in the operating state which minimizes the difference between the predicted current and an instructed current. Therefore, because the inverter is operated so as to optimize the variation of the predicted current, the problem described above can be avoided to a certain degree.
Published Japanese Patent First Publication No. 2006-174697 also proposes a control similar to this model prediction control.
However, the inventor of this application found out that, because the operating state is set in one control period of time by predicting a current flowing through the motor in the next control period of time, the current actually flow through the motor cannot be controlled with high precision. To control the current under this model prediction control with high precision, it is necessary in the present control period to provisionally determine one operating state, to be set in each of several control periods subsequent to the present control period, to predict a current of the motor for each of the provisionally-determined operating states, and to finally determine the operating state, to be set in the control period subsequent to the present control period, from the predicted currents. However, in the case of this prediction, the load of calculation on a computer is extremely increased.